Diesel fuels constitute a broad class of petroleum products and include distillate or residual materials (or blends of these two) from the refining of crude oil and are typically used in compression ignition or diesel engines. The two primary criteria used to define a diesel fuel are the distillation range (generally between 150xc2x0 and 380xc2x0 C.) and specific gravity range (between 0.760 and 0.935 at 15xc2x0 C.). The properties of diesel fuel greatly overlap those of kerosene, jet fuels, and burner fuel oils and thus all these products are generally referred to as intermediate distillates.
The flash-point is the lowest temperature at which vapors from a petroleum product such as diesel fuel will ignite on application of a small flame under standard test conditions. The approximate boiling range of diesel fuel is 150-380xc2x0 C. The lower end of this range (vapor pressures of hydrocarbons) tends to correlate with typical flash-point temperatures of greater than 38xc2x0 C. (greater than 56xc2x0 C. in some European countries) for diesel fuel. Due to safe handling and storage considerations and a legal flash-point requirement for diesel fuel, it is desirable to control the flash-point of diesel fuel blend.
The cetane number of a diesel fuel is roughly analogous to the octane number of gasoline. A high cetane number indicates the ability of a diesel engine fuel to ignite quickly after being injected into the combustion cylinder.
Particulate matter consists essentially of very small particles of carbonaceous soot and results from the incomplete combustion of fuel droplets. Particulate matter is subject to strict regulatory restrictions.
The cloud point is the temperature at which a sample of a petroleum product just shows a cloud or haze of wax crystals when cooled under standard test conditions. The cloud point of a diesel fuel is important for its low temperature performance. All of the above four characteristics should be taken into account when blending a diesel fuel.
The present legal flash-point requirements are summarized below by geographic region:
U.S. Pat. No. 4,424,063 discloses diesel fuel additives which increase the flash-point of the resulting composition from 6 to 29xc2x0 C. These additives are elemental iron, methyl isobutyl ketone, picric acid, normal butyl alcohol, nitrobenzene and primenes (tertiary alkylamines). Methyl isobutyl ketone and normal butyl alcohol are oxygenated compounds. Normal butyl alcohol has an estimated cetane number between 12-18 based on other linear low molecular weight alcohols.
U.S. Pat. No. 4,818,250 discloses adding hydrogenated limonene (saturated paraffin) or distilled limonene with an anti-oxidant to diesel fuel. Both modifications prevent gum from forming in the diesel fuel blend. However, limonene in a 10/90 blend with diesel fuel lowers the cetane number from 51.5 (no limonene) to 48.0. The flash-point of limonene is 49xc2x0 C. The flash-point of the diesel fuel is 60xc2x0 C. In this example, limonene can be used as a partial substitute for diesel fuel, but it lowers the cetane number of the diesel fuel blend.
U.S. Pat. No. 5,858,030 discloses diesel fuel compositions for increasing cetane number by adding oxygenates of dimethoxypropane and dimethoxyethane or dimethoxyethane with methanol and dimethoxymethane to diesel fuel. In this example methanol and dimethoxymethane only have cetane numbers of 5 and 29, respectively.
The above patents fail to teach a method which is capable of simultaneously and independently adjusting flash-point and cetane number of the final diesel fuel and possibly to also decrease particulate matter emissions.
Researchers are looking for a method which can be universally applied to produce a diesel fuel blend from any available stock diesel fuel wherein the diesel fuel blend can be prepared to possess any desired flash point and cetane number in order to meet the fuel properties required in any given locality.
Furthermore, researchers are looking for a method to control the flash-point of a diesel fuel blend utilizing blending components which improve the performance of the diesel fuel blend. An increase in performance can be measured by the increase of cetane number of the blended fuel, a decrease in particulate matter, negligible effect on cloud point and miscibility over a wide temperature range.
The present invention, which overcomes the disadvantages of prior art, relates to a method of producing a diesel fuel blend having a pre-determined flash-point and a pre-determined increase in cetane number, comprising the steps of:
a) selecting a stock diesel fuel with a flash-point and a cetane number,
b) establishing the pre-determined flash-point and the pre-determined increase in cetane number of the diesel fuel blend to be produced;
c) adding an amount of a first oxygenate with a flash-point less than the flash-point of said stock diesel fuel and a cetane number equal to or greater than the cetane number of said stock diesel fuel, said amount being sufficient to adjust the flash-point of the diesel fuel blend to the pre-determined flash-point; and
d) adding an amount of a second oxygenate with a flash-point equal to or greater than the flash-point of said stock diesel fuel and a cetane number greater than the cetane number of said stock diesel fuel, said amount being sufficient to achieve the pre-determined increase in cetane number
wherein the first oxygenate and the second oxygenate are not the same oxygenate.
To adjust the final flash-point the amount of the first oxygenate can according to a first embodiment be determined by the equation
T1/T2=1+T1RIn[x]/xcex94H 
wherein
T1 is the flash-point temperature of the first oxygenate,
T2 is the pre-determined flash-point temperature of the diesel fuel blend,
R is the ideal gas constant,
xcex94H is the enthalpy of vaporization of the first oxygenate, and
[x] is the mole fraction of the first oxygenate in the diesel fuel blend.
In another embodiment of the present invention, the amount of the first oxygenate to adjust the final flash-point is obtained from a calibration curve established by measuring the final flash-point of various mixtures of the stock diesel fuel and the first oxygenate.
The amount of the second oxygenate to adjust the final cetane number can be obtained from a calibration curve established by measuring the final cetane number of various mixtures of the stock diesel fuel and the second oxygenate.